Up to recently, the old brands T-Mobile and Orange have been available at EE stores.

Now, EE tries to push all T-Mobile/Orange customers to EE. They don't sell the old 3G plans anymore. Though old customers can stay on their Orange and T-Mobile brands. This is the end for Orange and T-Mobile in the UK. In effect, they have given up the multi-brand strategy, to become a single-brand.

I think this is a risky strategy. A lot of customers are happy with 3G only. EE practically charges extra for its plans which are all on 4G through higher prices. This can be a chance for more MVNOs to get new customers who want 2G/3G only at low prices and don't need 4G/LTE.

Not surprising, since BT is buying EE, and it will have nothing to do with its former parent companies any more.

This is a first in Europe so far: The market leader (EE) now sells only plans enabled with 4G/LTE for prepaid.
Given the fact, that they have a coverage of 80% for LTE now, while its competitors are at around 50%, this seems plausible.

From a MNO perspective, it's an experiement nobody dared in Europe so far:
The no.1s in France (orange), Netherlands (kpn), Spain (movistar), Austria (a1) or Germany (telekom) have not given access to 4G/LTE to prepaid customers up to now. There, they are the most restrictive. In these countries the smaller MNOs opened LTE for prepaid.
EE is clearly taking a very different approach in the UK.